One of the primary steps in the fabrication of modern semiconductor devices is the formation of a layer, such as a silicon oxide layer, on a substrate or wafer. As is well known, such a layer can be deposited by chemical vapor deposition (CVD). In a conventional thermal CVD process, reactive gases are supplied to the substrate surface where heat-induced chemical reactions take place to form the desired film. In a conventional plasma CVD process, a controlled plasma is formed using, for example, radio frequency (RF) energy or microwave energy to decompose and/or energize reactive species in reactant gases to produce the desired film.
Unwanted deposition on areas such as the walls of the processing chamber also occurs during such CVD processes. As is known in the industry, it is common to remove the unwanted deposition material that builds up on the interior of chamber walls with an in situ chamber clean operation. Common chamber cleaning techniques include the use of an etchant gas, such as fluorine, to remove the deposited material from the chamber walls and other areas. In some processes, the etchant gas is introduced into the chamber and a plasma is formed so that the etchant gas reacts with and removes the deposited material from the chamber walls. Such cleaning procedures are commonly performed between deposition steps for every wafer or every n wafers.
Some semiconductor manufactures employ a remote plasma cleaning process as an alternative to an in situ plasma cleaning, a remote plasma cleaning procedure may be employed in which an etchant plasma is generated remote from the substrate processing chamber by a high density plasma source such as a microwave plasma system, toroidal plasma generator or similar device. Dissociated species from the etchant plasma are then transported to the substrate processing chamber where they can react with and etch away the undesired deposition build up. Remote plasma cleaning procedures are sometimes used by manufacturers because they provide a “softer” etch than in situ plasma cleans, i.e., there is less ion bombardment and/or physical damage to chamber components because the plasma is not in contact with chamber components.
In one known type of remote plasma cleaning procedure, nitrogen trifluoride (NF3) is introduced into a remote plasma system (RPS) where it is activated by microwave power. The RPS dissociates the NF3 into reactive fluorine groups (e.g., radical F atoms and ions) that are transported to the substrate processing chamber to react with the residual deposition materials (e.g., silicon oxide) that have built up on the chamber sidewall and other exposed surfaces in the deposition chamber. The RPS system is often mounted on an external surface of the deposition chamber (e.g., the top of the chamber) and flows the activated cleaning gas into the chamber.
The activated cleaning gas may include the activated NF3 source gas to which a carrier gas such as helium or argon (Ar) may optionally be added. The rate at which the activated cleaning gas flows from the RPS into the deposition chamber is often limited by the construction of the RPS. For example, an ASTRONe RPS, manufactured by MKS Instruments Inc., is rated for a 4.0 SLM flow while an ASTRONex RPS system is rated for a 6.0 SLM flow. To keep the flow of the activated cleaning gas circulating through the chamber, a foreline is kept open to connect the chamber to an exhaust (e.g., dry) pump. The vacuum pulled by the dry pump causes cleaning gas to exit the chamber through the foreline.
In the 300 mm Ultimata HDP-CVD chamber, manufactured by Applied Materials, the portion of the foreline through which cleaning gases are exhausted is coupled to a single port running between the chamber and a roughing pump. The port is of a fixed size and has a limited flow capacity that cannot accommodate increased input flows of the cleaning gas beyond a certain point without an increase in the chamber pressure. Thus, when the ASTRON RPS units referred to above are used with the 300 mm Ultima chamber, a flow rate of the activated cleaning gas was typically in the range of between 2 to 4.5 Standard Liters per Minute (SLM). At such flow rates chamber pressure can readily be kept within an ideal range for efficient cleaning. When a higher flow RPS unit that can generate flow of activated cleaning gas in the range of 10 to 15 SLM, the single port foreline cannot remove the gas at a fast enough rate and the chamber pressure rises above an ideal range resulting in a decrease in the cleaning efficiency of the activated cleaning gas. For example, when the cleaning gas pressure climbs above about 9 Torr, more gas is used and the cleaning rate actually decreases compared to a lower chamber pressure. This limitation on the input flow rate of cleaning gas results in longer chamber cleaning times and reduces its throughput or productivity.